Abstract

This work investigates the ability of Coanda jet blowing to modify the base pressure of a cylindrical body aligned axially in a flow, and thereby, produce overall drag reduction. It is found that blowing through one or two slot jets concentric to the outer body circumference can significantly influence the entire base flow region. The recirculating wake is eliminated and is replaced by freestream fluid entrained by the Coanda blowing. Base pressure rises significantly and leads to drag reduction of up to 30% beyond the thrusting action of the Coanda jet. A comparison between the power savings through drag reduction and the power requirement of the Coanda jet demonstrates that net benefits are attainable at certain body geometries and flow conditions. By judiciously selecting the jet blowing velocity, it is possible to produce a nearly flat wake velocity profile requiring little net power. RAG reduction of immersed bodies is a subject with a long history, leading to the early concept of streamlining. The net drag force on a body may be considered the sum of viscous drag and pressure drag forces. For streamlined aero- dynamic and hydrodynamic bodies, the pressure drag is small, and current research is directed towards the application of laminar flow control (e.g., suction), turbulent viscous drag reduction (e.g., riblets and large eddy breakup devices (LEBUs)), and the use of nonlinear aerodynamics for induced drag reduction (e.g., winglets or crescent wing planforms). For bluff bodies, on the other hand, streamlining is usually not an option for reducing drag because the bluff shape is often dictated by other constraints. This situation is particu- larly true for trucks, buses, and most automobiles. For bluff bodies then, where pressure drag dominates, drag reduction is primarily through base flow modification, including flow separation control using airfoils, the use of plates, cavities, base bleed, and suction/blowing. Bluff bodies may be considered of two general classifica- tions—high and low aspect ratio. The former may be modeled as bodies of large span relative to a characteristic height, and are generally two-dimensional in nature. Examples would be the classic cylinder in crossflow, or a symmetric airfoil with a thick, blunt trailing edge. Drag reduction for such bodies has been performed13 with drag reductions of up to 64% reported in the literature. Low aspect ratio bodies have also been studied. Such bodies are characterized by three-dimensionality or axisymmetry, with a sphere being the classic example. Other examples would be cylinders of rectangular or circular cross section aligned axially in the flow direction. Work on drag reduction of such bodies has been reported in Refs. 4-12. A variation of these studies would include inclined base regions such as fast-back auto- mobiles and cargo transport aircraft. During the energy crisis of the seventies, renewed interest in vehicle drag led to many important new findings which are summarized in the books

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